1. Field of the Invention
This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
2. Description of Related Art
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle or a tundish into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and tend to produce defects caused by uneven solidification at the chilled casting surfaces of the rolls. Much attention has therefore been given to the design of metal delivery nozzles aimed at producing a smooth even flow of metal to and within the casting pool.
It has previously been proposed to introduce the molten metal into the casting pool by means of a metal delivery nozzle projecting downwardly into and formed as an elongate trough with openings in its longitudinal side walls. In use, molten metal flows into the trough and thereafter into the molten metal pool via the openings in the longitudinal side walls in two mutually oppositely directed series of jet streams which are directed outwardly to impinge on the casting rolls. One example of a metal delivery nozzle of this kind is disclosed in the applicants"" Australian Patent Application 60773/96.
The applicants have found metal delivery nozzles to be a particularly effective means of controlling the flow of molten metal into the molten metal pool.
In a commercial casting operation, molten metal will be delivered to a casting station in ladles and supplied to a twin roll caster either directly via the ladles or indirectly via a tundish. In practice, due to physical constraints it is probable that there will be a minimum gap of the order of 1 m between the outlet nozzle of a ladle or tundish and a metal delivery nozzle in the twin roll caster, with the consequence that molten metal will flow under high pressure from the ladle or the ladle/tundish assembly into the metal delivery nozzle unless an intermediate flow distributor is used such as that detailed in the applicants"" Australian Patent Application 59352/94. Such devices, although successful, create additional cost, particularly through the requirement that they be refurbished after each use.
The term xe2x80x9ctundishxe2x80x9d as used herein, except in relation to the description of the preferred embodiment, is understood to mean any vessel which holds and feeds molten metal to a twin roll caster and includes, but is not limited to, vessels that are known by the terms xe2x80x9cladlexe2x80x9d and xe2x80x9ctundishxe2x80x9d. In the description of the preferred embodiment the term xe2x80x9ctundishxe2x80x9d is used in its normal context.
In view of the relatively small size of the metal delivery nozzle, the entry of molten metal at high pressure is likely to cause substantial undesirable splashing of molten metal from the metal delivery nozzle and damage to the metal delivery nozzlexe2x80x94particularly in the areas where the molten metal impinges directly on the metal delivery nozzle.
Japanese Patent Publication 1-5650 of Nippon Steel Corporation discloses a submerged entry nozzle as an alternative for supplying molten metal to a metal delivery nozzle of a twin roll caster. The metal delivery nozzle has outlets that supply molten metal into a casting pool in mutually oppositely directed streams towards the casting rolls. The submerged entry nozzle is of conventional configuration and is positioned so that the outlets direct molten metal into the metal delivery nozzle in streams that are parallel to the longitudinal axis of the rolls.
The applicants have carried out a water modelling programme with a conventional submerged entry nozzle positioned as described in Japanese Patent Publication 1-5650, ie with the outlets arranged to direct water flow parallel to casting rolls. In the programme, the submerged entry nozzle was positioned in a metal delivery nozzle of the type described in Australian Application 60773/96. The applicants were not able to develop satisfactory flow patterns within the delivery nozzle to supply water to the openings in the longitudinal side walls of the metal delivery nozzle. In addition, the applicants have found that the arrangement of the submerged entry nozzle and the metal delivery nozzle produced substantial splashingxe2x80x94which is undesirable.
An object of the present invention is to alleviate the disadvantages described in the preceding paragraph.
According to the present invention there is provided a twin roll caster for casting molten metal, the twin roll caster comprising;
(a) a pair of parallel casting rolls forming a nip between them;
(b) an elongate metal delivery nozzle disposed above and extending along the nip between the casting rolls for supplying molten metal to a casting pool of molten metal between the rolls, the metal delivery nozzle having a bottom wall, longitudinal side walls which extend parallel to the axes of the rolls, end walls, and outlets for molten metal in the side walls;
(c) an entry nozzle for supplying molten metal to the metal delivery nozzle, the entry nozzle having an inlet end for receiving molten metal and an outlet end for supplying molten metal into the metal delivery nozzle, the outlet end extending into the metal delivery nozzle and having a bottom wall, elongate side walls spaced inwardly of the side walls of the metal delivery nozzle, and end walls, and outlets for molten metal in the side walls; and
(d) a tundish for supplying molten metal to the entry nozzle at the inlet end.
According to the present invention there is also provided a method of casting metal strip comprising, introducing molten metal between a pair of parallel chilled casting rolls via an entry nozzle of the type described in the preceding paragraph extending into an elongate metal delivery nozzle disposed above and extending along the nip between the rolls to form a casting pool of molten metal supported above the nip, and rotating the rolls to cast a solidified strip downwardly from the nip.
The outlets for molten metal in the entry nozzle may be in any suitable form, such as holes and slots.
The number and size of the outlets in the entry nozzle may be selected as required to suit particular casting requirements.
The main objective of the outlets in the entry nozzle is to enable the creation of optimum flow patterns of molten metal in the metal delivery nozzle. The optimum flow patterns in any given casting operation will depend on a range of factors including but not limited to the composition of the molten metal being cast.
It is preferred that the side walls of the entry nozzle be parallel to the side walls of the metal delivery nozzle.
It is also preferred that the outlets for molten metal in the entry nozzle are not laterally aligned with outlets of the delivery nozzle so that molten metal cannot flow directly from one outlet to the other.
The entry nozzle may comprise outlets for molten metal in its end walls.
The delivery nozzle may also comprise outlets for molten metal in its end walls.
It is preferred that the twin roll caster further comprises a ladle for supplying molten metal to the tundish
It is preferred that the twin roll caster further comprises a control means, such as a sliding gate valve or a stopper rod, for controlling the flow rate of molten metal from the tundish into the entry nozzle.
It is also preferred that the metal delivery nozzle be an upwardly opening elongate trough extending longitudinally of the nip between the casting rolls to receive molten metal, the bottom wall of the trough being closed, and the outlets for molten metal in the longitudinal side walls comprising a series of horizontally spaced openings in each respective side wall.
According to the present invention there is also provided a method of starting-up casting with a twin roll caster, the caster comprising a pair of parallel casting rolls forming a nip between them, an elongate metal delivery nozzle disposed above and extending along the nip between the casting rolls for supplying molten metal into the nip, an entry nozzle for supplying molten metal to the metal delivery nozzle, and a tundish for supplying molten metal to the entry nozzle, the method comprising preheating to a temperature of at least 1000xc2x0 C. the tundish, the metal delivery nozzle and the entry nozzle, positioning the preheated metal delivery nozzle relative to the casting rolls so that it is in its position disposed above and extending along the nip, fitting the preheated entry nozzle to the bottom of the preheated tundish, and lowering the tundish toward the delivery nozzle such that the entry nozzle extends into the delivery nozzle to enable the supply of molten metal from the tundish to the metal delivery nozzle via the entry nozzle.
The metal delivery nozzle may be positioned relative to the rolls before the entry nozzle is fitted to the tundish.
Alternatively, the entry nozzle may be fitted to the tundish before the delivery nozzle is positioned relative to the rolls and the tundish subsequently lowered to cause the entry nozzle to enter the delivery nozzle.